1. Field of the Invention
The present invention generally relates to a method and apparatus for aligning control zones in sheetmaking systems.
2. State of the Art
In modem automated paper making machines, continuous paper webs, sometimes measuring as much as four-hundred inches across, can be produced at rates of up to one-hundred feet per second. To control the quality of the paper manufactured at such rates and to reduce the quantity of finished product that must be rejected if there are upsets in the manufacturing process, properties of the paper web must be measured and adjusted while the machines are operating. That is, quality measurements and controls must be made on-line.
On-line measurements in sheetmaking manufacturing processes can be used for various purposes. For example, the on-line measurements can be employed as a basis for adjusting pulp feedstock composition, changing the quantity of steam or other heat applied to a paper web during production. The on-line measurements also can be employed as a basis for varying the nip pressure between calendaring rollers at the end of a papermaking process. In each of these examples, the on-line measurements are employed as a basis for operating control devices that affect sheet quality at various stages in a manufacturing process.
The control devices that are associated with modem papermaking machines include, for instance, feedboxes, steamboxes, and calendaring systems. Feedboxes operate to discharge the feedstock that forms the paper web. Steamboxes control the quantity of heat applied to the web. In a calendaring system, the calendaring rollers apply pressure at selected zones across the paper web.
On-line measurements in papermaking and other sheetmaking machines typically are categorized as either in the machine direction or in the cross direction. Machine-direction measurements reveal the sheet quality variation along the direction that a web travels during manufacture. Cross-directional measurements, on the other hand, reveal the variation across a web surface in a direction which is perpendicular to the machine direction.
Cross-directional measurements are typically made by scanning sensors that periodically move back and forth across the width of a travelling paper web. The sensors can be ones that measure sheet properties such as moisture content, caliper (i.e., thickness), and basis weight (i.e., mass per unit surface area). In the case of basis weight, travelling webs of sheet material are often measured by beta gauges that comprises a source of beta radiation (beta rays) and an ionization chamber. In operation, beta gauges are mounted on a sheetmaking machine in a position such that the beta rays are directed against one surface of a traveling web of sheet material while the ionization chamber is located for detecting the beta rays that have passed through the web from the source. The quantity of beta rays that are absorbed or transmitted over a given area of sheet material is a measure of the basis weight of the sheet material.
For conventional high-speed scanning sensors, a complete scan across a web typically requires between fifteen and thirty seconds. During a scan, measurements may be read from a sensor at intervals as frequent as fifty milliseconds. (Such intervals are referred to herein as minislice intervals.) A sequence of successive adjacent minislice measurements defines a measurement zone or slice.
In practice, control devices that are associated with sheet making machines normally include a series of actuator systems arranged in the cross direction. For example, in a typical feedbox, the control device is a flexible member or slice lip that extends laterally across a small gap at the bottom discharge port of the feedbox. The slice lip is movable for adjusting the area of the gap and, hence, for adjusting the rate at which feedstock is discharged from the feedbox. A typical slice lip is operated by a number of actuator systems, or cells, that operate to cause localized bending of the slice lip at spaced apart locations in the cross-direction. The localized bending of the slice lip member, in turn, determines the width of the feed gap at the various slices locations across the web.
It has been suggested that sheetmaking machines can be controlled by adjusting actuators using measurement signals provided by scanning sensors. In the case of cross direction control, for example, a commonly suggested control scheme is to measure values at selected cross direction locations on a sheet and then to compare those measured values to target or setpoint values. The difference for each pair of measured and setpoint values--the error--can be used for algorithmically generating appropriate outputs to cross direction control actuators to minimize the error. In such systems, a measurement zone is defined as the cross direction portion of sheet which is measured and used as feedback control for a cross direction actuator zone, and a control zone is defined as the portion of the sheet affected by a cross direction actuator zone.
In practice, it is difficult to control sheetmaking machines by adjusting actuators using measurement signals provided by scanning sensors. The difficulties particularly arise because the scanning sensors are separated from the control actuators by substantial distances in the machine direction. Because of such separations, it is difficult to determine which measurements zones are associated with which actuator zones. Such difficulties are referred to as alignment problems in the papermaking art. Alignment problems are exacerbated when, as is typical, there is uneven paper shrinkage of a paper web as it progresses through a papermaking process.
Another difficulty is that the effect of each actuator is not always limited within the corresponding control zone but spans over a few control zones. Knowing this effect is normally referred to an interzone coupling. The precise characteristics of the coupling effect and accomplishing algorithmic decoupling accordingly is almost as knowing crucial as the proper alignment.
One conventional method in papermaking for aligning actuator zones with measurement zones involves the use of dye tests. In a dye test, narrow streams of colored liquid are applied to feedstock as it flows beneath a slice lip. The dye streams initially form parallel lines that extend in the machine direction, but those lines may deviate from parallel if there is web shrinkage during the papermaking process. The dye marks passing through the measurement devices reveal the distribution of control zones and therefore specify the alignment of measurement zones.
Conventional dye tests, however, have numerous drawbacks. The most serious drawback is that the tests destroy finished product and, therefore, it is seldom feasible to perform dye tests at an intermediate point in a sheetmaking production run, even though sheetmaking processes are likely to drift out of control during such times. Further, because of the limited thickness and high absorption characteristics of tissue grades of paper, dye tests are typically limited to paper products that have relatively high weight grades.